This knowledge review tells us what works in integrating early years services. It is based on a rapid review of the research literature involving systematic searching, analysis of key data, validated local practice examples and views from people using services and providers. It summarises the best available evidence that will help service providers to improve services and, ultimately, outcomes for children, young people and their families. The Institute of Education carried out this review on behalf of the Centre for Excellence and Outcomes in Children and Young People’s Services (C4EO). The National Foundation for Educational Research (NFER) conducted the data analysis.